In general, the term “organic light-emitting phenomenon” refers to a phenomenon in which electrical energy is converted to light energy by means of an organic material.
An organic light-emitting device using the organic light-emitting phenomenon has a structure usually including an anode, a cathode, and an organic material layer interposed therebetween. In this regard, the organic material layer may have, for the most part, a multilayer structure consisting of different materials, for example, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injection layer. In the organic light-emitting device having such a structure, application of a voltage between the two electrodes injects a hole from the anode and an electron from the cathode to the organic layer. In the luminescent zone, the hole and the electron recombine to produce an exciton. When the exciton returns to the ground state from the excited state, the molecule of the organic layer emits light. Such an organic light-emitting device is known to have characteristics such as self-luminescence, high luminance, high efficiency, low driving voltage, a wide viewing angle, high contrast, and high-speed response.
The materials used as organic layers in organic light-emitting devices may be divided into luminescent materials and charge-carrying materials, for example, a hole injection material, a hole transport material, an electron injection material, and an electron transport material. As for the luminescent materials, there are two main families according to molecular weight: those based on small molecules and those employing polymers. The light-emitting mechanism forms the basis for classification of the luminescent materials as fluorescent or phosphorescent materials, which use excitons in singlet and triplet states, respectively. Further, luminescent materials may be divided according to color into blue, green, and red light-emitting materials. Further, yellow and reddish yellow light-emitting materials have been developed in order to achieve more natural colors.
Meanwhile, when a single material is employed as the luminescent material, intermolecular actions cause the wavelength of maximum luminescence to shift toward a longer wavelength, resulting in reduced color purity and light emission efficiency due to the light attenuation. In this regard, a host-dopant system may be used as a luminescent material so as to increase the color purity and the light emission efficiency through energy transfer.
This is based on the principle whereby, when a dopant is smaller in energy band gap than a host accounting for the light-emitting layer, the addition of a small amount of the dopant to the host generates excitons from the light-emitting layer so that the excitons are transported to the dopant, emitting light at high efficiency. Here, light of desired wavelengths can be obtained depending on the kind of dopant because the wavelength of the host moves to the wavelength range of the dopant.
In order for organic light-emitting devices to sufficiently exhibit the aforementioned outstanding properties, materials accounting for organic layers in the devices, for example, hole injection materials, hole transport materials, light-emitting materials, electron transport materials, electron injection materials, etc., should be based on stable and effective materials in advance.
Application of an electric current to such an organic light-emitting device induces the injection of holes and electrons from the anode and the cathode, respectively. After being transported respectively by a hole transport layer and an electron transport layer, the injected holes and electrons recombine in a light-emitting layer to produce excitons. The excitons return to the ground state, emitting light. According to the light-emitting mechanism, the light is classified as fluorescence emission with singlet transition to singlet and phosphorescence emission with triplet transition singlet. The fluorescence and the phosphorescence may be used as luminescent light sources of organic light-emitting devices.
In fluorescent organic light-emitting devices, only the formation of singlet excitons results in the emission of useful radiation, and thus there is a theoretical limit of 25% in the internal quantum efficiency of fluorescent organic light-emitting devices. On the other hand, phosphorescent light, which uses triplet excitons, has been extensively studied because its emission efficiency is far superior to that of fluorescent light.
So far, the most widely known phosphorescent host material is CBP, and organic light-emitting devices employing a hole barrier layer of BCP, BAlq, etc. are also known.
However, although devices employing phosphorescent materials are higher in terms of efficiency than those employing fluorescent materials, conventional phosphorescent host materials, such as BAlq or CBP, have room for improvement because they require high driving voltages and are unsatisfactory in terms of lifespan.
With regard to related arts pertaining to such phosphorescent materials for use in light-emitting devices, reference may be made to Korean Patent Publication No. 10-2011-0013220 A (Feb. 9, 2011), which discloses an organic compound having a 6-membered aromatic or heteroaromatic ring frame grafted with an aromatic heterocyclic ring, and Japanese Patent Publication No. 2010-166070 A (Jul. 29, 2010), which discloses an organic compound having a substituted or unsubstituted pyrimidine or quinazoline frame grafted with an aryl or heteroaryl ring.
Despite enormous efforts to prepare luminescent materials for use in organic light-emitting devices or electron transport materials, there is still a continued need to develop organic light-emitting devices that exhibit higher light emission efficiency and which can be driven at low voltages.